Elixir: Elixir (1.5.1) recompiles the deps code everytime after I edited my code

[neroishero]

Environment

• Elixir & Erlang versions (elixir 1.5.1)， opt 20
• win7 64

Current behavior

step:

1. "iex -S mix debug_client"
2. change some code of server.ex
3. try "iex -S mix debug_client" again,
   then you can sell the console recompile the chronos app, logger_file_backend app, decimal app

Here is the bug test repository: https://github.com/neroishero/elixir_test

[josevalim]

I could not reproduce it locally. @overminddl1 @OnorioCatenacci could one of you please give it a try on Windows? thank you!

[OnorioCatenacci]

I'll see if I can get to it. I don't have Win 7; have Win 10 but I'll see if I can repro it on that platform.

[OnorioCatenacci]

I checked this on Windows 10. Cannot reproduce the behavior. Couple of thoughts for @neroishero:

How do you exit iex (step 1)? I do a System.halt() in iex to insure everything is shut down.

What sort of change do you make in server.ex? Can you specifically tell us what kind of change you make? I just added a comment which did trigger recompilation but not of every file.

Are you sure that everything compiled correctly for step 1? If something didn't get compiled in step 1 that would cause the behavior you're seeing.

As I say, I know this is Win 10 not Win 7 and that may make a difference so we need someone with Win 7 to try to reproduce this but it'd also help if you could provide those details I spelled out.

[OnorioCatenacci]

Just to double check, I also tried modifying server.ex without shutting down iex. I then did a recompile inside iex. Same behavior--it only recompiled the proper files.

[OvermindDL1]

I checked this on Windows 10. Cannot reproduce the behavior. Couple of thoughts for @neroishero:

Similar here, which again makes me think it is something system specific like a file syncher or antivirus or so.

[vans163]

Would it be possible to add a compiler option to mix that would make it hash every file instead of checking the utime + file size?

[robi-wan]

I could not reproduce it under Windows 7 Pro 64bit.
Between the two iex sessions I edited this line: https://github.com/neroishero/elixir_test/blob/master/lib/service/server.ex#L75 (I increased the time).
I ended the first iex session with CTRL+C.
Session log is here.

[vans163]

@robi-wan its likely neroishero has an antivirus, malware or some filesystem that messes with the files utimes. Hence why reproducing it outside their environment is unlikely.

[josevalim]

Since multiple users have reported they could not reproduce the issue, I will go ahead and close this issue. Thank you for reporting and please follow up on the suggestions mentioned here. Thanks everyone who investigated and followed up. :+1:

[vans163]

Using utimes is clearly problematic for deciding whether to recompile deps, its funny how no one wants to admit this.

[josevalim]

At the same time mtimes+size is by far the fastest approach to this. I believe someone was working on file hashing and we are definitely open to such contributions. But even if added hashing to Mix, I don't think it would replace mtimes+size as the default choice for projects.

[vans163]

It might be the fastest but whats faster 1ms ping or 3ms ping, at this level of speed it really does not matter. The only problem I can see is if your project has thousands of source files (most users would have under 100), even then on an SSD it should be quite speedy. HDD's might/would suffer.

[josevalim]

Sure, if we can validate the assumption that the slowdown is not significant, then we can consider completely moving away from mtimes. But until then, someone needs to provide the hash-based implementation and perform the relevant benchmarks.